The invention relates to a radio-frequency amplifier circuit which is controlled by setting an operating point.
Electronic components and assemblies are subject to fluctuations and tolerances. Fluctuations in the technological parameters in manufacture lead to tolerances in the characteristic values of components. For example, the DC (direct current) gain B and the small-signal gain xcex2 of bipolar transistors are subject to manufacturing tolerances. If the components are subject to stringent accuracy requirements, manufacturing tolerances have to be reduced by complex trimming measures.
Assemblies formed from components which are subject to tolerances are in consequence themselves subject to fluctuations. For example, the characteristic of a power amplifier depends on the characteristic of the internal transistors. In an amplifier which is formed from field-effect transistors, the control characteristic of the power amplifier depends, for example, on the threshold voltage UTO of the field-effect transistors. The threshold voltage UTO of the field-effect transistors is dependent on manufacturing tolerances, as is their transconductance or gradient. For small dimensions, the threshold voltage is dependent, for example, on the channel length and channel width of the field-effect transistor.
Radio-frequency power amplifiers, are used, for example, as transmit amplifiers in systems which operate using the timeslot method (Time Division Multiple Access, TDMA method). One example of a timeslot method such as this is the GSM (Global System for Mobile Communications) mobile radio standard. In methods such as this, the data are transmitted broken down into so-called time slices. To ensure that the output power of the radio-frequency amplifier is zero when it is in the rest state, an amplifier which controls the amplifier must set the control input of the radio-frequency amplifier to a voltage below the threshold voltage of the amplifier transistor. However, this has a disadvantageous influence on the characteristics of the radio-frequency amplifier. There is a dead zone from the regulator quiescent voltage, which is applied to the input of the amplifier, to the inception of the power from the amplifier at the beginning of a transmit timeslot. This dead zone causes a deterioration of the control characteristics of the system.
If the control characteristics of the system are too poor, trimming must be carried out. It is known for such trimming to be carried out on an individual basis, for example using a potentiometer, or for the reference voltage of the regulator to be modified, for example by superimposing an offset. However, implementations of such individual trimming options require additional outlay and are costly.
The need for trimming the control of a power amplifier can also arise as a result of changing environmental conditions, for example, temperature, or as a result of a change in the supply voltage.
It is accordingly an object of the invention to provide an amplifier circuit which overcomes the above-mentioned disadvantages of the heretofore-known circuits of this general type and which has a compensation for manufacturing tolerances.
With the foregoing and other objects in view there is provided, in accordance with the invention, an amplifier circuit, including:
a power amplifier having a first transistor to be supplied with a supply voltage, the first transistor having a control input for receiving an input signal and a load connection for supplying an output signal;
a control amplifier having an operational amplifier, the operational amplifier having an input and an output, the input being connected to the load connection of the first transistor, the output being connected to the control input of the first transistor; and
a reference stage having a second transistor to be supplied with the supply voltage, the second transistor being connected to the operational amplifier for supplying a reference voltage to the operational amplifier.
In other words, the object of the invention is achieved by an amplifier circuit having a power amplifier with a first transistor to which a supply voltage can be supplied, to whose control input an input signal can be supplied and at whose first load connection an output signal can be tapped off, a control amplifier having an operational amplifier whose output is connected to the control input of the first transistor and whose first input is connected to the first load connection of the first transistor, and a reference stage having a second transistor to which the supply voltage c an be supplied and which is connected to the operational amplifier so that a reference voltage can be supplied to this operational amplifier.
A first transistor which can be connected to a supply voltage is specified as the power amplifier. This amplifier is supplied on the input side with an input signal which is to be amplified, and the amplified input signal can be tapped off as the output signal on the output side. This power amplifier is controlled by a control amplifier. The control amplifier has an operational amplifier whose output is connected to the input of the first transistor. Furthermore, the output signal which can be tapped off on the amplifier is fed back to a first input of the operational amplifier. The power amplifier forms the control element for this regulator configuration.
In order to form a reference voltage, a second transistor is provided in a reference stage and is connected to the operational amplifier in order to transmit a reference voltage value. This second transistor is a physical equivalent of the first transistor, so that it can be used for tolerance compensation. The first and second transistors are of the same transistor type, but the second transistor is scaled with respect to the first transistor. The first and second transistors have, in particular, the same electrical characteristics.
A reference signal of the regulator can be supplied to a second input of the operational amplifier via a filter and governs the desired output power from the power amplifier. Together with the control amplifier, the second transistor (which is provided in the reference stage) forms a compensation circuit for the radio-frequency power amplifier, so that the control process can operate without trimming.
In comparison to once-off trimming, the present circuit has the additional advantage that the temperature and operating voltage dependency of the amplifier circuit is also covered and automatically compensated for by the reference stage. Such a temperature change can occur, for example, as a result of operation, or can result from external influences. When the circuit is used as a transmit amplifier in the mobile radio area, the operating voltage depends, for example, on the charge state of the rechargeable battery in the mobile radio appliance.
In one advantageous embodiment of the invention, the power amplifier and the reference stage are provided on the same semiconductor chip. This firstly ensures good matching of the transistors during manufacture so that the first and second transistors have characteristics which differ as little as possible from one another and, secondly, the placement on the same chip allows close thermal coupling.
Thus, with the objects of the invention in view there is also provided, a chip configuration, including:
a semiconductor chip having a power amplifier and a reference stage disposed thereon;
the power amplifier including a first transistor to be supplied with a supply voltage, the first transistor having a control input for receiving an input signal and a load connection for supplying an output signal;
the reference stage including a second transistor to be supplied with the supply voltage; and
a control amplifier having an operational amplifier, the operational amplifier having an input and an output, the input being connected to the load connection of the first transistor, the output being connected to the control input of the first transistor; and
the second transistor being connected to the operational amplifier for supplying a reference voltage to the operational amplifier.
According to another feature of the invention, a first capacitor is connected to the control input of the first transistor such that the input signal can be supplied via the first capacitor; and a second capacitor is connected to the load connection of the first transistor such that the output signal can be derived with the second capacitor.
According to yet another feature of the invention, a third capacitor is connected between the output of the operational amplifier and the second input of the operational amplifier.
According to a further feature of the invention, the first and second transistors are field-effect transistors of the same transistor type.
According to another feature of the invention, the first and second transistors have substantially identical transconductances and substantially identical threshold voltages.
According to yet another feature of the invention, a high-impedance reference resistor is connected to the second transistor.
According to a further feature of the invention, the first and second transistors are bipolar transistors of the same transistor type.
According to another feature of the invention, wherein the first and second transistors have substantially identical static current gains and substantially identical differential current gains.
In a further, advantageous embodiment of the present invention, the threshold voltage (which is subject to manufacturing tolerances) of the first transistor can be trimmed by the control amplifier in that the second transistor, which is provided in the reference stage, has a high resistance to ground at its source connection, across which virtually all the threshold voltage is dropped. This reference voltage is used, via a connection between the reference stage and the operational amplifier, to compensate for tolerances in the threshold voltage of the first transistor.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an amplifier circuit, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.